1. Field of the Invention
Generally, the present invention relates to metrology in the manufacturing of microstructures, such as integrated circuits, and, more particularly, to the measurement of the surface characteristics of microstructure features by means of SPM (scanning probe microscopy) metrology tools with modulated surface excitation, which allow the determination of surface and near-surface characteristics with nanometer resolution.
2. Description of the Related Art
In manufacturing microstructures, such as integrated circuits, micromechanical devices, opto-electronic components and the like, device features such as circuit elements are typically formed on an appropriate substrate by patterning the surface portions of one or more material layers previously formed on the substrate. Since the dimensions, i.e., the length, width and height of individual features, are steadily decreasing in order to enhance performance and improve cost-effectiveness, these dimensions have to be maintained within tightly set tolerances in order to guarantee the required functionality of the complete device. Usually, a large number of process steps have to be carried out for completing a microstructure, and, thus, the dimensions of the features during the various manufacturing stages have to be thoroughly monitored to maintain process control and to avoid further cost-intensive process steps owing to process tools that fail to meet the specifications in an early manufacturing stage.
For example, in highly sophisticated CMOS devices, the gate electrode, which may be considered as a polysilicon line formed on a thin gate insulation layer, is an extremely critical feature of a field effect transistor and significantly influences the characteristics thereof. Consequently, the size and shape of the gate electrode has to be precisely controlled to provide the required transistor properties. Thus, great efforts are being made to steadily monitor the dimensions of the gate electrode.
However, for highly scaled microstructures, the surface characteristics of the materials used increasingly become important aspects for enhancing performance and reliability of the devices and/or for reducing yield loss and the like. For example, the detection of grain sizes, micro-cracks, adhesion characteristics and elastic properties may be important for the performance and reliability of the devices, especially when highly scaled microstructure devices are considered. By way of example, in metallization layers of advanced integrated circuits, the adhesion characteristics may significantly affect the mechanical strength during the manufacturing process and may also determine the reliability of the finished device while the crystallinity of the metal also influences the current drive capability and thus the performance of the device. Moreover, the formation of well-understood interfaces between two different materials is an important aspect and may therefore require reliable measurement data of surface characteristics.
Consequently, there was a growing need for techniques which may be able to characterize materials and in particular their engineered surfaces with a spatial resolution that is appropriate for highly scaled microstructures, such as integrated circuits. A frequently used and very powerful tool in this respect is the atomic force microscope (AFM) which allows the characterization of nano-structured materials by scanning an appropriate tip of a cantilever across a sample surface. During the scan operation, typically the charge cloud of the tip interacts with respective charge clouds of the sample surface, wherein the corresponding interaction, i.e., the minimal displacements of the tip, are recorded to obtain information on the surface structure with a nanometer resolution. Thus, atomic force microscopy provides a three-dimensional image of the surface topography, which may provide precious information with respect to the surface structure of the sample. However, when imaging a sample surface with high spatial resolution, respective surface portions within an area of a few micrometers or smaller may appear almost atomically flat so that the corresponding image obtained by means of the atomic force microscope includes less contrast and thus may not allow the extraction of detailed information on surface characteristics at this high resolution, although, on a broader scale, a significant sample topography may be present.
Therefore, recently, a new technique has been developed, in which image contrast and thus extraction of details of small areas of interest may be efficiently enhanced by exciting the sample surface with sound waves, wherein ultrasonic sound with frequencies up to several MHz may be used. A corresponding excitation of the sample surface is typically used in combination with an atomic force microscope and this technique is typically referred to as ultrasonic force microscopy (UFM). Using this technique, the tip of the atomic force microscope is used to detect the acoustic or ultrasonic waves via the sample surface, wherein elastic changes of the surface below the tip permits extraction of detailed information on the elastic properties of the sample surface with high spatial resolution.
With reference to FIG. 1, a typical conventional metrology tool will now be described to explain the basic principle of UFM for the investigation of sample surfaces, such as layer surfaces as used during the fabrication of microstructural features. In FIG. 1, a system 100 comprises a sample holder 101, which may be provided to include a piezoelectric material that is connected to a modulation unit 103 which may provide modulated signals to the sample holder 101 so as to exert vibrational excitation and thus a surface excitation to a sample 102 placed on the substrate holder 101. The substrate holder 101 may further be configured to allow a scan operation in at least two dimensions, which may be substantially perpendicular to the surface portion of the sample 102 to be examined. The system 100 further comprises a tip 104 as a scanning probe, which may interact with the surface of the sample 102. The tip 104 may be attached to a corresponding cantilever, which in turn is supported by a cantilever holder 105, which may also comprise an appropriately configured piezo crystal that may be controlled by a specific PI (proportional integral) controller 106. Thus, the tip 104 may be biased by the cantilever holder 105 so as to adjust the force with which the tip 104 contacts or interacts with the sample 102. In other configurations of microscopes of the scanning probe type, the tip 104 and the corresponding cantilever holder 105 may be configured to provide the scan functionality. That is, the tip 104 is scanned across the sample 102, while the sample 102 may remain fixed. Moreover, a laser 107 may be positioned such that the output beam thereof impinges on the cantilever and is reflected therefrom so as to be detected by a corresponding optical detector 108, so that the output signal of the detector 108 is a measure for the displacement of the tip 104. The detector 108 is connected to lock-in amplifier 109, which is also connected to the modulation unit 103, thereby enabling one to tune the lock-in amplifier 109 to a specified frequency component with the intention to reduce signal noise and to facilitate information extraction from the output signal of the detector 108. Moreover, a data processor 110 may be provided to manipulate the data obtained from the lock-in amplifier 109, which may then be displayed or otherwise provided in a display unit 111.
During the operation of the system 100, the sample 102 is positioned on the substrate holder 101 and is scanned in one or two dimensions, while the displacement of tip 104, biased with a specific force as controlled by the PI controller 106, is detected by the detector 108, the signal of which is processed by the lock-in amplifier 109. During the measurement procedure, a specific modulation signal 112 is supplied to the substrate holder 101 to excite the sample, and thus the sample surface, with a specified signal containing frequencies up to several MHz, wherein these frequencies may be modulated with a corresponding carrier wave, such as a sawtooth wave and the like. During the scan operation, the output signal of the detector 108 is picked up by the lock-in amplifier 109, which is tuned to the modulation frequency, thereby obtaining information on the power of a modulation frequency component in the signal spectrum provided by the detector 108. This signal may further be used by the data processor 110 to provide an image, that is, a qualitative mapping of the UFM response to exciting the sample 102, with a high spatial resolution compared to pure AFM techniques. Consequently, surface characteristics, such as crystallinity, elastic properties and the like, may be qualitatively investigated and monitored with high spatial resolution.
It appears, however, that for a quantitative determination of specific nanomechanical properties, such as highly precise values of elastic properties of surface portions, the conventional UFM system 100 may suffer from a plurality of problems. For instance, the UFM signal may be strongly modified by signal artifacts, which may originate from the PI controller 106 that provides the bias force of the tip 104 during the scan operation. Since the lock-in amplifier 109 is tuned to the modulation frequency, such substantially DC or low frequency interferences may not be identified by the data processor 110, thereby resulting in highly unreliable measurement data. Moreover, the data processor 110 receives information about the power of a modulation frequency component but does not receive any information about the waveform of the signal itself. Consequently, in sophisticated applications, a precise and reliable determination of surface-related characteristics may be difficult with conventional systems, even if a conventional UFM tool is capable of providing a high spatial resolution.
In view of the above-described situation, a need exists for an enhanced technique that avoids or at least reduces one or more of the problems identified above.